noun (n)

• someone who assists in a plot(noun.person)

  Synonym:
  confederate, henchman, partner in crime
  

  source: wordnet30
• someone who collaborates with an enemy occupying force(noun.person)

  Synonym:
  collaborationist, quisling
  

  source: wordnet30
• an associate in an activity or endeavor or sphere of common interest(noun.person)

  Example:
  The musician and the librettist were collaborators.
  

  Synonym:
  cooperator, pardner, partner
  

  source: wordnet30
• An associate in labor, especially in literary or scientific labor.(noun)

  source: webster1913